Current VOD systems face a number of challenges. One of them is how to provide the clients, which may be in the number of millions, with sufficient interactivity like fast-forward/backward and/or forward/backward-jump. At the same time, the provision of such functions should not impose severe network load, as the network resources namely the bandwidth may be limited. Furthermore, every client generally prefers to have the movie he selects to be started as soon as possible.
The following sections describe some of the currently used VOD systems and their possible disadvantages:                1. Near-VOD (NVOD) with regular stream-interval        
A NVOD system consists of staggered multicast streams with regular stream interval T (FIG. 1). The streams are multiplexed onto the same or different physical media for distribution to the users via some multiplexing mechanisms (such as time-division multiplexing, frequency division multiplexing, code-division multiplexing, wavelength division multiplexing etc. . . ). The distribution mechanisms include point-to-point, point-to-multipoint and other methods. Each stream is divided into regular segments of interval T, and the segments are labelled 1, 2, 3, . . . , N respectively. The content that is to be distributed to the users is carried on the N segments and the content is replicated on all these streams. The content is also repeated on each stream in time. By using such a staggered streaming arrangement with regular stream interval T, the users are guaranteed to receive the content at any time with a start-up latency less than T. However, there is no provision for user interactivity in such a system. If a user interrupts the content viewing say by pausing the display, the user cannot resume the viewing at the same play point where the user pauses and is forced to skip some content to keep up with the multicast-stream that is continuously playing.                2. Quasi-VOD (QVOD) with irregular stream-interval        
A QVOD system consists of staggered multicast streams with irregular stream intervals (FIG. 2). The streams are multiplexed onto the same or different physical media for distribution to the users via some multiplexing mechanisms (such as time-division multiplexing, frequency division multiplexing, code-division multiplexing, wavelength division multiplexing etc. . . ). The distribution mechanisms include point-to-point, point-to-multipoint and other methods. Unlike the NVOD system where the streams constantly exist, the streams in a QVOD system are created on demand from the users' request for the content. The users' requests within a certain time interval Ti are batched together and served together by Stream i. The stream intervals T1, T2, . . . Ti, . . . are irregular. The streams (Stream 1 to i etc. . . ) are all provided on-demand and will be removed as soon as the content distribution has been completed. The streams are constantly created as users' requests come in. By using such a staggered streaming arrangement with irregular stream interval Ti, the particular group of users starting within interval Ti is guaranteed to receive the contents within ti (start-up latency). Again, there is no provision for user interactivity in such a system. If a user interrupts the content viewing say by pausing the display, the user cannot resume the viewing at the same play point where the user pauses and is forced to skip some content to keep up with the multicast-stream that is continuously playing.                3. Distributed Interactive Network Architecture (DINA)        
DINA system refers to the method and system as described in the applicant's PCT applications PCT/IB00/001857 & 001858. In the DINA system, interactive functions including fast-forward/backward, forward/backward-jump, slow motions, and so on can be provided by a plurality of multicast video data streams in conjunction with a plurality of distributed interactive servers. Although interactive functions may be provided to the client in such the DINA system, the network load may increases if the start-up time for each user's request is to be reduced. This is determined by the stream interval of the multicast data streams. Generally, the number of data streams, and therefore the network load, increases with the decrease of the stream interval.
In the NVOD and QVOD Systems, a user wanting to view the content will simply tap into one of the many staggered streams and view the content simultaneously with all others sharing the stream. While such schemes are simple and efficient, they suffer from two difficulties—a large start-up latency and user inflexibility.
For the first difficulty, a user may have to wait as long as one stream interval T before the request is served, and the waiting time may be as large as many minutes or even hours, depending on the stream interval. Although the stream interval can be made very small, say even down to a few seconds, this also means that the system has to provide a large number of streams for serving the same amount of content. The number of streams required is simply
      R    T    ,where R is the length of the content and T is the stream interval. Thus, small start-up latency may incur a much higher transmission bandwidth and cost. The DINA system may also face such a difficulty.
For the second difficulty, the users viewing a multicast stream cannot freely interrupt the stream because there are other viewers. Therefore, NVOD and QVOD systems cannot allow VCR-liked interactivity such as pause, resume, rewind, slow motion, fast forward, and so on. These systems also hinder the introduction of new forms of interactive media to be deployed. In recent years, one popular approach to offer some form of VCR-liked interactivity over NVOD and QVOD systems is to add a storage unit to the set top box (STB) so as to cache all the available content being broadcast. Such systems suffer from a higher system cost and operational problems like storage unit failure and management.
It can be realised that the prior art may fail to provide a solution to the existing problems in VOD systems. Specifically, current VOD systems may not be able to provide the clients/users with desired interactive functions with a short start-up time, while at the same time minimising the network load. Therefore, it is an object of this invention to resolve at least some of the problems at set forth in the prior art. As a minimum, it is an object of this invention to provide the public with a useful choice.